Method for revamping a conventional mineral oils refinery to a biorefinery

ABSTRACT

The invention also relates to the transformation unit of mixtures of a biological origin obtained with said conversion method and particularly hydrodeoxygenation and isomerization processes.

The present application is a divisional of U.S. patent application Ser.No. 14/425,501 filed on Apr. 30, 2015, which is a 35 U.S.C. § 371national stage patent application of international patent applicationPCT/IT12/000268, filed on Sep. 4, 2012 and which claims priority fromM12012A001465 filed on Sep. 3, 2012. The disclosures of saidapplications are hereby incorporated by reference in their entirety.

A method is described for revamping a conventional refinery of mineraloils into a biorefinery, characterized by a production scheme whichallows the treatment of raw materials of a biological origin (vegetableoils, animal fats, exhausted cooking oils) for the production ofbiofuels, prevalently high-quality biodiesel.

This method allows the re-use of existing plants, allowing, inparticular, the revamping of hydrodesulfurization plants into productionplants of hydrocarbon fractions which can be used as diesel fuel, or asdiesel fuel components, starting from a mixture of a biological origincontaining triglycerides possibly with aliquots of free fatty acids.Together with diesel fuel, the plants deriving from the reconversion ofdesulfurization plants also produce aliquots of naphtha and LPG, whichare consequently also products deriving from mixtures of a biologicalorigin.

Through this method, it is possible to re-use existing equipment ofhydrodesulfurization units by means of a rearrangement which allows toobtain a new configuration, suitable for carrying out processes for theproduction of fuel bases from biological mixtures, in particular diesel:said rearrangement is capable of providing the same efficiency as aplant specifically constructed for this process, with reduced costs.

Furthermore, as the hydrodesulfurization unit is normally inserted in arefinery context, its rearrangement to a transformation unit ofbiological feedstocks into diesel fuel allows to exploit products andby-products of the same refinery in the transformation process ofbiological material, by integration of the refinery units producing themwith the new configuration and the new use of the hydrodesulfurizationunit, also enabling the use of all the auxiliary services normallypresent in a refinery.

The use of vegetable oils in diesel engines goes back to Rudolf Diesel,who, in 1900, demonstrated the capacity of diesel engines of functioningwith peanut oil. During the second world war, both palm oil and peanutoil were used in Africa as fuel for military vehicles. After the war,technological development led to an almost exclusive use of fuelsderiving from petroleum; in addition, diesel engines were enormouslyimproved, mainly with respect to the injectors and control systems, tosuch an extent that there was little flexibility for the use of fuelsdifferent from gasoil. At the same time, vegetable fuels wereprogressively abandoned due to the high production cost and inconstancyin the product quality.

During the oil crisis of the seventies', attention was refocused on theuse of vegetable oils as diesel fuels, but this was difficult forvarious reasons (formation of crusting in the combustion chamber,blockage of the injectors, dilution of the lubricant). Researchactivities were therefore directed towards the preparation, startingfrom vegetable oils, of methyl or ethyl esters and their use in dieselengines. Methyl and ethyl esters of fatty acids are obtained fromvegetable oils by transesterification with methanol or ethanol.

An alternative approach for the conversion of vegetable oils wasproposed in the eighties' and consists in their deep hydrogenation toproduce hydrocarbon fractions with a boiling point compatible withdiesel fuels obtained from mineral oil. The deep hydrogenation ofvegetable oils causes the removal of oxygen with the contemporaneousformation of a mixture of H₂O, CO₂ and CO, in reciprocal ratios varyingaccording to the operative conditions. The starting components are thusprevalently transformed into hydrocarbons with respect to triglycerides,and also fatty acids and glycerin. Small quantities of free alcohols canbe formed together with the hydrocarbons.

The deep hydrogenation reaction of fatty oils to produce liquid fuelswas studied for example, again in the eighties', by Nunes et al., who,in the article entitled “Hydrocraquage sous pression d'une huile desoja: procédé d′étude et allure generale de la transformation” (Rev.Inst. Fr. Pet. Of 1086, vol. 41, page 421 onwards) describes thehydrocracking of soya oil with a bifunctional catalyst. At temperaturehigher than 673 K, decarbonylation and decarboxylation of the fattyacids are observed, together with a strong hydrogenolysis due to thepresence of the metallic catalyst. The main products are linear-chainhydrocarbons.

J. Gusmao et al. (Utilization of vegetable oils as an alternative sourcefor diesel-type fuel: hydrocracking on reduced Ni/SiO₂ and sulphidedNi—Mo/Al₂O₃, Catalysis Today 5 of 1989 page 533 onwards) demonstrateshow, in the hydrogenation of soya oil, the hydrocarbon fraction obtainedmainly consists of linear paraffins (96% molar of C₁₅-C₁₆-C₁₇-C₁₈).

U.S. Pat. No. 4,992,605 describes a process for producing hydrocarbonfractions in the C₁₅-C₁₈ range by the hydrogenation of vegetable oilssuch as sunflower oil, rape oil, canola oil, palm oil, or fatty oilscontained in the pulp of pine trees (tall oil). This hydrocarbonfractions prevalently consists of linear paraffins (C₁₅-C₁₈) and ischaracterized by a high cetane number, which is such that it can be usedas a cetane improver.

In “Hydroprocessed vegetable oils for diesel fuel improvement”,Bioresources Technology 56 (1996), pages 13 to 18, the applicationdescribed in U.S. Pat. No. 4,992,605 is summarized, on a laboratoryscale to produce a hydrogenated product starting from canola oil.

EP 1396531 describes a process for the production of hydrocarboncomponents from mixtures of a vegetable or animal origin. The formationof a mixture with a content of iso-paraffins of 73% is described. Theprocess comprises a pre-hydrogenation step, a hydrodeoxygenation step(HDO) and an isomerization step which operates using the countercurrentflow principle.

EP 1728844 describes a process for the production of hydrocarboncomponents from mixtures of a vegetable or animal origin. The processcomprises a pretreatment step of the mixture of a vegetable origin forremoving contaminants, such as, for example, alkaline metals, followedby a hydrodeoxygenation (HDO) step and possibly an isomerization step.

EP 2084245 describes a process for the production of a hydrocarbonmixture that can be used as diesel fuel or diesel component by thehydrodeoxygenation of a mixture of a biological origin containing fattyacid esters possibly with aliquots of free fatty acids, such as forexample vegetable oils such as sunflower oil, rape oil, canola oil, palmoil, or fatty oils contained in the pulp of pine trees (tall oil),followed by hydroisomerization on specific catalysts, which allows toobtain hydrocarbon mixtures in which the content of isoparaffins canexceed 80%, the remaining percentage being n-paraffins.

Current regulations require that fuel components from renewable sources,for example from mixtures of a biological origin containing fatty acidesters, be present in fuels for a percentage of around 4.5% (referringto calorific value) for 2012, which will be equal to 5.0% within 2014and will reach 10% in 2020, according to Dlg. nº 28 of 2011 whichimplements the Europe Directive 2009/28/CE.

The biological diesel component which is currently used in most cases isFAME (Fatty Acid Methyl Ester) i.e. a mixture of methyl esters of fattyacids deriving from the transesterification with methanol oftriglycerides contained in vegetable oils. For as much as it is widelyused, FAME has disadvantages from a quality point of view, due to thelow calorific value (about 38 kJ/kg) and poor cold properties (Cloudpoint from −5° C. to +15° C.)

As FAME is miscible with water, moreover, it can cause pollution intanks, it has a low stability, tends to polymerize forming rubbers andother undesired products, causes fouling, thus dirtying of the filters,and dissolves in lubricating oil. For these reasons, various automobilecompanies are advising against the use of FAME in their engines. Thisresults in the possibility of using FAME in a limited maximum quantitywhich is such as to not satisfy the standards required by the directive2009/28/CE (RED) for the promotion of the use of energy from renewablesources and the directive 2009/30/CE (FQD) for product quality.

There is therefore the necessity of producing higher-quality componentsfor diesel of a biological origin, and of consequently increasing theproduction of diesel of a biological origin, in short times, inparticular using technologies which produce higher-quality components.New dedicated plants are therefore required for facing the necessity ofincreasing the production capacity of high-quality biologicalcomponents.

In view of the construction of new plants, which requires lengthy timesand high investments, above all for the reactors, that must operate witha high hydrogen pressure, compressors and other machines and for theconstruction of a hydrogen production plant, the necessity is stronglyfelt for finding alternative solutions which allow the exploitation ofexisting production units by conversion of pre-existing plants, with theleast possible invasiveness and as economically as possible.

A method has now been found for transforming hydrodesulfurization unitsinto conversion units of mixtures of a biological origin, based ontriglycerides, into biocomponents for fuels, in particular for dieseland possibly jet fuel, LPG and gasoline: the new method is based onappropriate variations in the configuration of units already existingwith the selection of equipments that can be converted rather thanmodified and a limited number of substitutions and new installations.This method is of particular interest within the current economiccontext which envisages a reduction in the demand for oil products andrefinery margins, allowing the production cycle to be modified throughthe transformation of already-existing hydrodesulfurization units in oilrefineries into production units of hydrocarbon mixtures that can beused as fuels from mixtures of a biological origin.

This conversion of mixtures of a biological origin into biocomponentsconsists in the production of hydrocarbon fractions from mixtures of abiological origin containing triglycerides, by means of theirhydrodeoxygenation and isomerization: said conversion is indicatedhereunder with the name “HDO/ISO process”. The “HDO/ISO process”therefore refers to a process for producing, as main product, ahydrocarbon fraction which can be used as diesel fuel, or as diesel fuelcomponent, starting from a mixture of a biological origin containingfatty acid esters, and possibly containing free fatty acids, whichcomprises the following steps:

-   -   1. Hydrodeoxygenation of the mixture of a biological origin;    -   2. Hydroisomerization of the mixture resulting from step (1),        after a possible purification treatment.

In said HDO/ISO process, the mixture of a biological origin is a mixtureof a vegetable or animal origin, and the fatty acid esters containedtherein are fatty acid triglycerides, wherein the hydrocarbon chain ofthe fatty acid contains from 12 to 24 carbon atoms and is mono- orpoly-unsaturated. The mixtures of a biological origin can be selectedfrom vegetable oils, vegetable fats, animal fats, fish oils or mixturesthereof: the vegetable oils or fats, possibly deriving from plantsselected by genetic manipulation, are selected from sunflower, rape,canola, palm, soya, hemp, olive, linseed, mustard, peanut, castor,coconut oils, fatty oils contained in the pulp of pine trees (tall oil),oils extracted from seaweeds, recycled oils or fats of the food industryand mixtures thereof, and the animal oils or fats are selected fromlard, tallow, milk fats, recycled oils or fats of the food industry andmixtures thereof.

In the HDO/ISO process, the hydrodeoxygenation HDO step is carried outin the presence of hydrogen and a hydrogenation catalyst containing acarrier and one or more metals selected from metals of group VIII andgroup VIB. Preferably the catalysts are previously sulfided, by means ofthe known techniques. In order to keep the catalyst in sulfided form,the sulfiding agent, for example dimethyldisulfide, is fed incontinuous, contemporaneously with the feedstock, in a percentageranging from 0.02-0.5% weight (140-3400 ppm S).

The hydrodeoxygenation HDO step is normally done at a pressure rangingfrom 25 to 70 bar and at a temperature ranging from 240 to 450° C.

In the HDO/ISO process, the mixture of a biological origin can besubjected to a pretreatment before being fed to the HDO step (1),wherein said pretreatment can be effected by adsorption, treatment withion exchange resins or mild acid washings.

The mixture resulting from the HDO step (1) is subjected to apurification treatment before being subjected to hydroisomerization,wherein the purification treatment comprises a separation step and awashing step, in particular the mixture resulting from step (1) can besent to a high-pressure gas-liquid separator in order to recover agaseous phase and a liquid phase.

The gaseous phase, containing hydrogen, water, CO, CO₂, light paraffins(C4⁻) and small quantities of NH₃, PH₃ and H₂S, is cooled: uponcondensation, the water and condensable hydrocarbons are separated, andthe remaining gaseous phase is purified to obtain hydrogen that can berecycled to the reaction step (1). The liquid phase separated in thehigh-pressure separator, composed of a hydrocarbon fraction, essentiallyconsisting of linear paraffins with a number of carbon atoms rangingfrom 14 to 21, is fed to the subsequent hydroisomerization step (2).

The hydroisomerization step (2) (ISO) can be effected at a temperatureranging from 250 to 450° C., and a pressure ranging from 25 to 70 bar.

Isomerization catalysts that can be conveniently used are catalystscontaining metals of group VIII, and a carrier selected, for example,among alumina or silica or silico-aluminas or zeolites. The metal ofgroup VIII is preferably Pt, Pd or Ni.

According to a particularly preferred aspect, according to what isdescribed in WO 2008/058664 and in WO2008/113492, a catalyticcomposition Me/MSA is used in the isomerization step, which comprises:

a) a carrier of an acid nature (NSA) comprising a completely amorphous,micro-mesoporous silico-alumina having a molar ratio SiO₂/Al₂O₃ rangingfrom 30 to 500, surface area greater than 500 m²/g, a pore volumeranging from 0.3 to 1.3 ml/g, an average pore diameter lower than 40 Å.

b) a metallic component (Me) comprising one or more metals of groupVIII, possibly mixed with one or more metals of group VIB.

The operating conditions, catalysts and preferred particular embodimentaspects of the HDO/ISO process are known to experts in the field, andare described, for example, in EP1396531, in EP 1728844, in EP 20884245,in WO 2008/058664, in WO2008/113492, and particular embodiments andutilizations of the HDO/ISO process are also described, for example, inWO2009/039347, WO2009/039335, WO2009/039333, WO2009/158268: all theaspects, operating conditions and catalysts described in these documentscan be used for carrying out the HDO/ISO process in the production unitof hydrocarbon fractions from mixtures of a biological origin obtainedby applying the method for revamping hydrodesulfurization unitsaccording to the present invention.

In accordance with what is specified above, an object of the presentinvention relates to a method for revamping a refinery comprising asystem containing two hydrodesulfurization units, U1 and U2, into abiorefinery comprising a production unit of hydrocarbon fractions frommixtures of a biological origin containing fatty acid esters by means oftheir hydrodeoxygenation and isomerization,

wherein each of the hydrodesulfurization units U1 and U2 comprises:

-   -   a hydrodesulfurization reactor, (A1) for the unit U1 and (A2)        for the unit U2, wherein said reactor contains a        hydrodesulfurization catalyst;    -   one or more heat exchangers between the feedstock and effluent        of the reactor;    -   a heating system of the feedstock upstream of the reactor;    -   an acid gas treatment unit downstream of the reactor, containing        an absorbent (B) for H₂S, said unit being called T1 in the unit        U1 and T2 in the unit U2, and wherein said method comprises:    -   installing a line L between the units U1 and U2 which connects        them in series;    -   installing a recycling line of the product for unit U1 and        possibly for the unit U2,    -   substituting the hydrodesulfurization catalyst in the reactor A1        with a hydrodeoxygenation catalyst;    -   substituting the hydrodesulfurization catalyst in the reactor A2        with an isomerization catalyst;    -   installing a by-pass line X of the acid gas treatment unit T2 of        the unit U2;    -   substituting the absorbent (B) in the acid gas treatment unit T1        with a specific absorbent for CO₂ and H₂S.

The term “refinery” normally refers to a complex of industrial plants inwhich the refining of oil, mineral oils or raw products of a petroleumorigin, effected. The refining is mainly oriented towards the productionof fuels. Said refinery is normally indicated as conventional refinery.

The term “biorefinery” refers to a complex of industrial plants in whichproducts and raw materials of a biological origin, such as, for example,vegetable oils, animal fats, exhausted cooking oils, are treated, toobtain fuels. The fuels thus obtained are generally indicated asbiofuels.

The hydrocarbon fractions that can be obtained from the biorefineryresulting from applying the method according to the present invention,are fuels or fuel components, in particular LPG, kerosene, diesel,naphtha.

The heating system of the feedstock upstream of the reactor ishereinafter called F1 for the unit U1 and F2 for the unit U2.

The new product recycling lines installed in the unit U1 and possiblythe unit U2 are hereinafter called R1 for the unit U1 and R2 for theunit U2.

A by-pass line of an apparatus refers to a line that passes around saidapparatus and consequently said apparatus is no longer used.

In particular, the by-pass line X passes around the unit T2 which istherefore no longer used.

The two hydrodesulfurization units to which the method of the presentinvention is applied can be hydrodesulfurization units normally used inparallel in common refinery schemes.

The heating system of the feedstock and the heat exchanger betweenfeedstock and effluent are different from each other.

The production of hydrocarbon fractions from mixtures of a biologicalorigin containing fatty acid esters by means of their hydrodeoxygenationand isomerization, corresponds to the “HDO/ISO process” previouslydescribed, and to all the particular embodiments described and known toexperts in the field. The unit of production of hydrocarbon fractionsfrom mixtures of a biological origin containing fatty acid esters bymeans of their hydrodeoxygenation and isomerization, is hereinaftercalled “HDO/ISO unit”. The term “unit” refers to the combination ofapparatuses for the embodiment of a process or treatment.

Desulfurization units that can be used for the method of the presentinvention are all units known to experts in the field: saiddesulfurization units comprise the hydrodesulfurization reactorcontaining the hydrodesulfurization catalyst, one or more heatexchangers between feedstock and effluent, a heating system of thestream at the inlet of the reactor, an acid gas treatment unitdownstream of the reactor, containing a specific absorbent for H₂S.

As is known to experts in the field, the hydrodesulfurization reactor isnormally made of low-bound carbon steel (for example 1¼ Cr-1/2 Mo, 2¼Cr-1 Mo) with respect to the reactor jacket, with a stainless steelinternal lining of the type 321 SS, 347 SS. The reactor interiors aregenerally made of stainless steel of the type 321 SS, according to whatis suggested by the standard API 941-2004. The hydrodesulfurizationreactors that can be used and their configurations are well-known toexperts in the field and are described for example in Handbook ofPetroleum Refining Processes, Robert A. Meyers.

The desulfurization catalysts are well-known to experts in the field,and can be selected from hydrogenation catalysts containing a carrier,normally alumina, and one or more metals selected from metals of groupVIII and group VIB, for example CoMo/Alumina; CoMo—NiMo/Alumina, and aredescribed, for example in “Petroleum Refining: Technology andEconomics”, of James H. Gary, Glenn E. Handwerk and “Hydrotreating andhydrocracking fondamentals, P. R. Robinson, G. E Dolbear.

The heat exchanger of the feedstock-effluent exchange train used in adesulfurization unit are normally made of low-bound carbon steel (1 Cr-½Mo) with possible inner coating of stainless steel (347 SS) or totallyof stainless steel (347Ss, 321 SS), for high-temperaturefeedstock-effluent exchangers, whereas it is made of simple orwork-hardened carbon steel (CS or KCS) for exchangers operating at lowertemperatures. These exchangers allow heat exchange between feedstock tothe reactor and its effluent. Heating systems, situated upstream of thehydrodesulfurization reactor and operating over the feedstock to thereactor can be selected from direct fired ovens and heat exchangers. Anoven comprising a radiating section and possibly a convective section ispreferably used.

The description of heating systems, and in particular ovens and theirconfigurations and production suitable for hydrodesulfurization unitscan be found, for example, in “Handbook of Petroleum Processing”, editedby David S. J. Jones and Peter P. Pujadó.

Acid gas treatment units suitable for being used in hydrodesulfurizationunits, their configurations and specific absorbents for the absorptionof H₂S are well-known to experts in the field and are described, forexample, in Selecting Amines for Sweetening Units, Polasek, J. (BryanResearch & Engineering) and Bullin, J. A. (Texas A&M University), GasProcessors Association Regional Meeting, September 1994.

An acid gas treatment unit refers to a system in which one or more gasesof an acid nature are separated from a gaseous mixture containing themby absorption with an absorbent and recovered by regeneration of theabsorbent.

Absorbents which can be used are, for example, solvents, preferably ofthe alkanol-amine type, for example MDEA (methyl-diethanol-amine) or DEA(diethanolamine).

One or more sulfur recovery units which can be used in the method of thepresent invention, are also normally present in refineries, as will bedescribed in more detail hereunder: said sulfur recovery units arewell-known to experts in the field and comprise a primary sulfurrecovery section, of the Claus type, and possibly a tail-gas treatmentsection suitable for increasing the conversion to sulfur. In particular,a sulfur recovery unit of the Claus type is composed of a first thermalreaction step, consisting of the furnace in which the acid gas is burntat temperatures higher than 1500° C. and where the Claus reaction takesplace (3H2S+(3/2)O2=>3S+3H2O), which converts about 70% by weight of thesulfur at the inlet of the unit, followed by a catalytic section,consisting of two or more catalytic reactors, containing an alumina bed,where part of the non-reacted H₂S is converted to elemental sulfur,alternated by an intermediate cooling suitable for condensing the sulfurproduced. A Claus unit thus formed reaches a recovery of about 96%-98%by wieght of the sulfur at the inlet. Said Claus unit and the catalystsused therein are well-known to experts in the field and are described,for example, in Fundamental and Practical Aspects of the Claus SulfurRecovery Process P. D. Clark, N. I. Dowling and M. Huang, Alberta SulfurResearch Ltd., Calgary, Alberta, Canada.

In the method of the present invention, the substitution of thehydrodesulfurization catalyst inside the reactors A1 and A2 with HDO andISO catalysts, respectively, does not involve any particular difficultyand can be easily effected. Said HDO and ISO catalysts can be selectedfrom known hydrodeoxygenation and isomerization catalysts, in particularthose previously described.

The units U1 and U2 also preferably contain hydrogen recycling lines,and relative compressors, which connect the acid gas treatment unitswhich are situated downstream of the reactors, with the same reactors:said lines, and relative compressors, are re-used for the same purposein the production unit of hydrocarbon fractions from mixtures of abiological origin obtained by the revamping method of the presentinvention.

A particularly preferred aspect relates to a method according to thepresent invention, additionally operating so as to recycle the H₂Sformed by the HDO step, after recovering it from the absorbent (B) ofT1. The H₂S is formed by decomposition of the sulfiding agents fed tothe HDO reactor A1 for maintaining the hydrodeoxygenation catalyst inits sulfided form, i.e. active form. In the HDO step, CO₂ is also formedby decarboxylation of the fatty acid esters.

As further described hereunder, in order to separate the H₂S, themixture of CO₂ and H₂S formed during the HDO step must be received fromthe absorbent of T1 and, after separating the H₂S from the CO₂, by meansof two additional absorption/regeneration steps, carried out in afurther acid gas treatment unit, called T3, the resulting stream of H₂Sfrom the HDO section is recycled, as sulfiding agent of the catalyst ofthe reactor A1, preferably sending it to the compressor K1 of thehydrogen recycling line of the unit U1 by means of a new line R3installed for this purpose.

In particular, the line R3 is connected to the suction of saidcompressor K1.

In accordance therefore with a preferred aspect, the method of thepresent invention also comprises the addition of said further acid gastreatment unit T3, downstream of the unit T1 and connected to said unitT1, in which the two above-mentioned additional absorption/regenerationsteps can be effected, and the installation of said new recycling lineof H₂S between the unit T3 and the HDO section, preferably in thesuction phase of the compressor K1 which is situated on the hydrogenrecycling line of the unit U1. The unit T3 contains two absorbing areaseach containing a specific absorbent for H₂S.

By applying, in accordance with the method of the present invention,said further modifications to the system comprising the units U1 and U2,the objective is reached of recycling the H₂S to the HDO reactor A1 inorder to maintain the catalyst in its sulfided form, guaranteeing itsactivity without the necessity of feeding other sulfiding agents, of thetype DMDS, or in any case feeding said sulfiding agents in a morelimited quantity.

A further advantage of said recycling of the H₂S lies in the substantialreduction in emissions of acid gas, which is reduced to CO₂ alone, andwhich consequently does not have to be treated in the Claus plant, notstrictly necessary unless there are other sources of H₂S, but can besent directly to the final thereto-combustor of the sulfur recoveryunit.

In accordance with what is specified above, a preferred aspect of thepresent invention therefore relates to a method for revamping a refinerycomprising a system containing two hydrodesulfurization units, U1 andU2, into a biorefinery containing a HDO/ISO unit comprising ahydrodeoxygenation section HDO and an isomerization section ISO,

wherein each of the hydrodesulfurization units, U1 and U2 comprises:

-   -   a hydrodesulfurization reactor, (A1) for the unit U1 and (A2)        for the unit U2, wherein said reactor contains a        hydrodesulfurization catalyst;    -   one or more heat exchangers between the feedstock and effluent        of the reactor;    -   a heating system of the feedstock upstream of the reactor;    -   an acid gas treatment unit downstream of the reactor, containing        an absorbent (B) for H₂S, said unit called T1 in the unit U1 and        T2 in the unit U2,        and wherein said method comprises:    -   installing a line L between the units U1 and U2 which connects        them in series;    -   installing a recycling line of the product for the unit U1 and        possibly for the unit U2,    -   substituting the hydrodesulfurization catalyst in the reactor A1        with a hydrodeoxygenation catalyst;    -   substituting the hydrodesulfurization catalyst in the reactor A2        with an isomerization catalyst;    -   installing a by-pass line X of the acid gas treatment unit T2 of        the unit U2;    -   substituting the absorbent (B) in the acid gas treatment unit T1        with a specific absorbent for CO₂ and H₂S;    -   installing an acid gas treatment unit T3 downstream of the acid        gas treatment unit T1 to separate the H₂S;    -   recycling the H₂S to the reactor A1.

The flow of H₂S at the outlet of T3 can reach the reactor A1 by means ofa new line that is connected in any point of the unit U1 suitable forthe purpose.

According to what is specified above, a particularly preferred aspect ofthe present invention relates to a method for revamping a refinerycomprising a system containing two hydrodesulfurization units, U1 andU2, into a biorefinery containing a HDO/ISO unit, wherein each of thehydrodesulfurization units U1 and U2 comprises:

-   -   a hydrodesulfurization reactor, (A1) for the unit U1 and (A2)        for the unit U2, wherein said reactor contains a        hydrodesulfurization catalyst;    -   one or more heat exchangers between the feedstock and effluent        of the reactor;    -   a heating system of the feedstock upstream of the reactor;    -   an acid gas treatment unit downstream of the reactor, containing        an absorbent (B) for H₂S, said units being called T1 in the unit        U1 and T2 in the unit U2,    -   a hydrogen recycling line and a compressor on said line, said        compressor being called K1 for the unit U1 and K2 for the unit        U2,        and wherein said method comprises:    -   installing a line L between the units U1 and U2 which connects        them in series;    -   installing a recycling line of the product for the unit U1 and        possibly for the unit U2,    -   substituting the hydrodesulfurization catalyst in the reactor A1        with a hydrodeoxygenation catalyst;    -   substituting the hydrodesulfurization catalyst in the reactor A2        with an isomerization catalyst;    -   installing a by-pass line X of the acid gas treatment unit T2 of        the unit U2;    -   substituting the absorbent (B) in the acid gas treatment unit T1        with a specific absorbent for CO₂ and H₂S;    -   installing an acid gas treatment unit T3 downstream of the acid        gas treatment unit T1;    -   installing a recycling line R3 of H₂S from the unit T3 to the        compressor K1 of the hydrogen recycling line of the unit U1,        preferably to the suction of the compressor K1.

As previously specified, one or more sulfur recovery units are normallypresent in refineries. These sulfur recovery units are well-known toexperts in the field and have already been described. In particular,sulfur recovery units of the Claus type are preferably used inrefineries: this type of unit has been previously described and, asalready specified, substantially comprises a thermal reaction sectionand a catalytic section.

A further particularly preferred aspect relates to a method according tothe present invention in which the unit U1 is connected to a sulfurrecovery unit, preferably a Claus unit, by the installation of a by-passline of the thermal section of said sulfur recovery unit, andsubstitution of the catalyst of the first reactor of the catalyticsection with a cold selective redox catalyst, capable of converting H₂Sinto liquid sulfur. Catalysts suitable for the purpose can be selectedfrom oxides of metals of group VIB combined with transition metals ofgroup VIII and are described for example in U.S. Pat. No. 6,372,193 andin the documents cited therein.

A further aspect of the present invention is to install a surge drum (S)upstream of each of the reactors A1 and A2.

The hydrodesulfurization units can contain, in addition to what hasalready been described above:

-   -   a line for feeding make-up hydrogen possibly after mixing it        with recycled hydrogen, to each of the reactors (A1) and (A2),        wherein said line can derive for example from the refinery        hydrogen network or directly from a reforming unit;    -   a fractionation unit of the products obtained, a gas        separation/washing unit, particularly fuel gas and gas rich in        propane which is formed during the process, downstream of each        of the reactors (A1) and (A2);        wherein all these equipments remain unvaried, do not undergo        changes or modifications due to the transformation method of the        present invention and are re-used as such, thus also being        included in the production unit of hydrocarbon fractions from        mixtures of a biological origin containing fatty acid esters by        means of their hydrodeoxygenation and isomerization, as said        unit results after application of the transformation method of        the present invention.

As previously specified, connection lines can be present between areforming unit and each of the reactors (A1) and (A2), as the reformingunit is normally present in a refinery together with the desulfurizationunit. Said lines, called (LH1) and (LH2), allow the hydrogen formed inthe reforming unit as by-product, to be used, whether it is a catalyticreforming unit or a steam reforming unit: said lines can therefore bepart, as such, of the production unit of hydrocarbons from mixtures of abiological origin: and be used without undergoing any modification.

The revamping of a refinery containing hydrodesulfurization unitsthrough the operations described above, allows to obtain, with minimuminterventions, times and costs, a biorefinery containing atransformation unit of mixtures containing fatty acid esters intohydrocarbon mixtures that can be used as diesel and diesel components bymeans of a process comprising a hydrodeoxygenation step and anisomerization step.

In particular, for carrying out the HDO/ISO process previouslydescribed, the hydrodeoxygenation step (HDO) is carried out in one ofthe hydrodesulfurization reactors, the reactor (A1), in which thestructure of the triglycerides contained in the mixture of a biologicalorigin is transformed into paraffinic compounds with the contemporaneousproduction of propane, CO₂ and water. As the HDO reaction is exothermic,the exothermicity is controlled by means of suitable product recycling,effected through a new line (R1) introduced downstream of the reactor:the product recycling, through a dilution effect, controls thetemperature increase in the same reactor. The exothermicity can also becontrolled with hydrogen quenching, which can already be normallypresent in the hydrodesulfurization units, alternating with the variouscatalytic beds. The new recycling line of the reaction product (R1) issuch as to allow a flow-rate equal to even double that of the freshfeedstock being fed to the reactor.

A surge drum (S) can be additionally inserted upstream to the reactorused for the hydrodeoxygenation step and upstream to the reactor usedfor the isomerization step (ISO): said drum has the purpose ofequalizing the feedstock, consisting of fresh feedstock plus reactedrecycled product.

The reactor used for effecting the HDO step, deriving, through themethod of the present invention, from a pre-existinghydrodesulfurization unit, may not have a metallurgy suitable forguaranteeing its use in the presence of high concentrations of freefatty acids in the feedstock consisting of a mixture of vegetable oils.The reactors of the HDO/ISO units specifically constructed for thispurpose, are in fact made of stainless steel (316 SS, 317 SS), to allowthem to treat contents of free fatty acids of up to 20% by weight of thefeedstock. The desulfurization reactor, produced with the typicalmetallurgy described above, can in any case be used for the treatment ofmixtures of a biological origin normally available on the market,containing free fatty acids in a quantity not exceeding the safetythreshold of 1,000 ppm by weight. If it is convenient to use mixtures ofa biological origin with a content of fatty acids higher than thisthreshold, a pretreatment of the feedstock suitable for reducing thiscontent can be possibly effected. If this treatment is used, apretreatment unit of the feedstock is added upstream of the unit A1, inorder to lower its content of free fatty acids.

The hydrocarbon product resulting from the HDO step is fed through thenew connection line (L) to the isomerization section where theisomerization step takes place, in the reactor A2, with possiblerecycling of the isomerization product to the same reactor A2, by meansof a new line (R2), in order to ensure the wettability of the catalyst,thus also allowing the use of a low quantity of fresh feedstock.

As previously specified, each of said hydrodesulfurization units towhich the revamping method of the present invention is applied, containsan acid gas treatment unit, hereinafter called acid gas washing unit,normally operating downstream of a high-pressure separator, situated onthe reactor effluent, whose function is to purify the hydrogen leavingthe reactor, by separation from the H₂S formed during thehydrodesulfurization, before said hydrogen is recycled. Furthermore, thedesulfurization unit normally contains a low-pressure separator fromwhich fuel gas (FG) is separated, containing methane, ethane and H₂S,whereas the liquid fraction is sent to a stripping column, suitable forseparating LPG and naphtha at the head and desulfurized gas oil at thebottom of the column. A vacuum dryer is normally inserted on the line ofthe desulfurized product for removing possible traces of water presentin the product, before sending it to storage. According to the method ofthe present invention, the new connection line (L) installed between thetwo units U1 and U2, is preferably inserted downstream of said dryingunit, if present in the unit U1, or it is inserted downstream of thestripping column of the unit U1.

As previously described, said high- and low-pressure separators, saidstripping column and said possible dryer remain unvaried, they do notundergo any changes or modifications due to the transformation method ofthe present invention and are re-used as such, thus also being able tobe a part of the production unit of hydrocarbon fractions from mixturesof a biological origin containing fatty acid esters by means of theirhydrodeoxygenation and isomerization, as said unit results afterapplication of the transformation method of the present invention.

Due to the different nature of the gases leaving the hydrodeoxygenationreactor, with respect to those generated by the hydrodesulfurization,for which the acid gas treatment unit was designed, hereinafter alsocalled acid gas washing unit, according to the method of the presentinvention, the absorbent used in the acid gas treatment unit must besubstituted: the gas leaving the reactor in which the HDO step iscarried out, in fact, mainly contains H₂, H₂S and CO₂, with a ratio ofabout 1-5% by weight of H₂S with respect to the total H₂S and CO₂,whereas in pre-existing hydrodesulfurization case, the gas leaving thereactor mainly contained H₂ and H₂S, with a high content of H₂S,resulting from the sulfur content of the raw material fed to therefinery.

The acid gas treatment unit therefore contains an absorbent (B) specificfor H₂S, normally a selective amine for H₂S. In the configurationderiving from the method of the present invention, as the reactorupstream of said acid gas separation unit is used for thehydrodeoxygenation of mixtures containing fatty acid esters, the gaseousby-product from which t hydrogen is to be purified before being recycledto the HDO reactor, is mainly CO₂, mixed with smaller quantities of H₂S,due to the continuous sulfidation of the HDO catalyst.

The varying nature, composition and flow-rate of the gases leaving theHDO reactor can be processed in the pre-existing acid gas treatment unitby simple substitution of the pre-existing absorbent suitable for theabsorption of H₂S with a selective absorbent for both CO₂ and H₂S.

In accordance with the method, object of the invention, the absorbent(B) of T1 is therefore substituted with an absorbent suitable forabsorbing both CO₂ and H₂S, the purified hydrogen is re-fed to thereactor and the gaseous mixture mainly containing CO₂ and H₂S (thelatter in a quantity normally ranging from 1 to 5% with respect to thesum of CO₂ and H₂S) is recovered from the absorbent used by means of aregeneration column, said column forming part of the pre-existing acidgas treatment system.

Absorbents suitable for the absorption of CO₂ and H₂S, in theproportions indicated above, and which can be used in the method of thepresent invention, are well-known to experts in the field. According toa preferred aspect, amines available on the market, produced by DOW andBASF, are used, and preferably methyldiethanolamine (MDEA) withpromoters or activated. Said amines are described, for example, in U.S.Pat. No. 6,337,059. Amines suitable for being used in the presentinvention, produced by DOW, are, for example, those of the seriesUCARSOL™ AP, such as, for example, AP802, AP804, AP806, AP810 and AP814, and preferably UCARSOL™ AP Solvent 810.

The mixture of CO₂ and H₂S is recovered from said absorbents byregeneration of the absorbent, particularly in the case of an aminesolvent, in a reboiled distillation column, operating at low pressure.Other impurities that may be present in the gases leaving the HDOreactor are removed by means of the same treatment described above.

According to another preferred aspect of the present invention, the H₂Spresent in the gas leaving the first reactor, A1, can be furtherconcentrated in order to be re-fed to said HDO reactor A1 to keep thecatalyst in sulfided form, guaranteeing its activity. According to saidpreferred aspect, the method of the present invention comprises theinstallation of a further acid gas treatment unit T3, downstream of thetreatment unit T1, in which two further steps are effected, each ofwhich comprises the absorption of a specific solvent, selective for H₂S,and relative regeneration. Said steps are suitable for separating theH₂S from the CO₂, present in the acid gas stream obtained from the acidgas treatment unit T1 of the unit U1, to obtain a stream of concentratedH₂S to be re-fed to the deoxygenation reactor A1 through the new line R3previously described, which connects T3 to the HDO section, preferablyto the compressor K1 of the hydrogen recycling line, and in particularto the suction of the compressor K1.

Absorbents suitable for the absorption of H₂S alone, which can be usedin the method of the present invention for the unit T3, are well-knownto experts in the field. According to a preferred aspect, aminesavailable on the market, produced by DOW and BASF, are used, andpreferably methyldiethanolamine (MDEA) with promoters or activated.Suitable amines, produced by DOW, are for example those of the seriesUCARSOL™ HS, such as, for example, HS101, HS102, HS103, HS104, HS115,and preferably UCARSOL™ HS solvent 102.

In the method of the present invention, as previously described, the useof the acid gas absorption section T2 installed on the recycled gas, isnot necessary in the ISO section, and for this purpose the by-pass lineX is installed: T2 can therefore possibly be re-used for one of the twoadditional separation steps, necessary for separating the H₂S from theCO₂ described above. The use of T2 is not necessary as the flow leavingthe reactor A2 does not contain H₂S.

The CO₂ is then recovered from the head of the two absorption columnsand sent to the final thermo-combustor of the sulfur recovery plant (ofthe Claus type) normally present in refineries, or to any refinery ovenpreviously equipped with specific flues on various burners for theintroduction of said stream.

According to another aspect of the method of the present invention, therefinery in which said method of the present invention is applied, isequipped with a sulfur recovery unit, wherein said unit has beenpreviously described and is preferably a Claus unit, which can be used,according to the method of the present invention, for treating theCO₂/H₂S gaseous mixture leaving the acid gas treatment unit according totwo possible modes, a traditional operative mode and a modifiedoperative mode.

The traditional operative mode, known to experts in the field and inaccordance with what has been previously described and indicated inFundamental and Practical Aspects of the Claus Sulfur Recovery ProcessP. D. Clark, N. I. Dowling and M. Huang, Alberta Sulfur Research Ltd.,Calgary, Alberta, Canada, is used when, in the refinery in which thehydrodesulfurization units U1 and U2 are present, there are other H₂Ssources such as to consider the contribution of the HDO/ISO processnegligible (for example hydrocrackers or other hydrodesulfurizations ofhydrocarbon fractions): when operating according to this mode, theCO₂/H₂S gaseous mixture is fed to the sulfur recovery unit, togetherwith the H₂S deriving from the other sources, wherein said sulfurrecovery unit is used as such, i.e. without undergoing any modification.According to this mode, the method of the present invention comprisesinstalling a connection line between T1 and the sulfur recovery unit.

The modified operative mode is suitable for treating low quantities ofacid gas with an extremely low H₂S content, as in the case in whichthere are no other significant H₂S sources in addition to the HDO/ISOprocess: this mode preferably uses a unit of the Claus type and iscarried out by the installation, from the unit T1, of a by-pass line ofthe hot section in the Claus unit (furnace) and by substitution of thecatalyst of the first of the Claus reactors of the sulfur recovery unitwith a catalyst suitable for the treatment of gaseous streams in whichH₂S is present in a concentration lower than 30% mole. Said catalyst canbe selected, for example, from oxides of metals of group VIB combinedwith transition metals of group VIII and are described, for example, inU.S. Pat. No. 6,372,193 and in the documents cited therein.

In this case, the unit for the treatment of tail gas possibly present,continues to operate to ensure a further abatement of SO₂ emissions.

The sulfur recovery is completed by the condensation of the liquidsulfur in the collection tanks normally included in a sulfur recoveryunit.

The hydrogen necessary for the production unit of hydrocarbon fractionsfrom mixtures of a biological origin, containing fatty acid esters, bymeans of their hydrodeoxygenation and isomerization, comprises recycledhydrogen and a flow of make-up hydrogen, preferably mixed with recycledhydrogen and fed to the reactors A1 and A2 used for the HDO step and ISOstep: said make-up hydrogen can be supplied, as previously indicated, byreforming units normally already present in refineries. In particular,heavy naphtha (BP 80-160° C.) can be fed to a catalytic reforming unit.

The reforming reaction conditions differ depending on the type of unitinstalled: for semigenerative reforming units, the operating pressure is16-28 barg with a Platinum-Rhenium catalyst and a H/C ratio of <4; forcontinuous new-generation reforming units, the operating pressure is2.5-5 bare with a Platinum-Tin catalyst and a H/C<3; the desired productis the reformate, a gasoline base with a high octane number (98-101),with the contemporaneous formation of H₂.

Natural gas, fuel gas, LPG or virgin naphtha are fed to the steamreforming unit; the steam reforming reaction takes place with a nickelcatalyst on alumina at high temperatures 750-900° C. and an operatingpressure of 20-40 barg. The desired product is H₂.

The hydrogen deriving from reforming can then fed to the reactors (A1)and (A2) by means of pre-existing lines or specific lines installed forthe purpose, respectively indicated as LH1, to the hydrodeoxygenationreactor A1, and LH2, to the isomerization reactor A2, and possibly afterpurification and concentration of the hydrogen flow by means of a PSAsystem. This configuration allows a separate and autonomous hydrogenfeed to be obtained for each reactor, thus improving the flexibility andoperability of the plants. This aspect represents an improvement withrespect to normal hydrodeoxygenation processes of vegetable oils, whichenvisage a single hydrogen circuit for the two reactors. The PSA(pressure swing adsorption) system, when present, uses for example aseries of beds filled with adsorbent material, typically a zeolite. Thestream of gas rich in hydrogen flows through the bed, the gaseousproducts are adsorbed and, as hydrogen has a lesser tendency to beadsorbed, a flow of pure hydrogen is obtained at the outlet of the PSAunit. The regeneration of the adsorbing bed must be cyclically effectedby depressurization.

A purification system of the feedstock of a biological origin can beadditionally added to the production unit of hydrocarbon fractions frommixtures of a biological origin, containing fatty acid esters, by meansof their hydrodeoxygenation and isomerization, obtained with thetransformation method of hydrodesulfurization unit of the presentinvention.

Said purification system is situated upstream of the reactor (A1) inwhich the HDO step is carried out and comprises a degumming, decoloring,deacidification and deodorizing unit. The purpose of this system is toremove the impurities that can poison the HDO catalyst, such as metals(P, Fe, Ca, Mg, K, Na) and nitrogen and reduce the content of free fattyacids.

FIG. 1 shows a scheme example relating to the HDO/ISO unit deriving fromthe revamping method of the present invention: the dashed parts andlines correspond to the new installations according to the method of thepresent invention, whereas the continuous parts and lines correspond tothe pre-existing hydrodesulfurization equipments.

In particular, in FIG. 1, Feed is the mixture of a biological originwhich is fed to the HDO/ISO unit, where said mixture can be, forexample, a refined vegetable oil, i.e. with a content of fatty acids nothigher than 1,000 ppm by weight.

The fresh feedstock of vegetable oil is fed through line 1 to the surgedrum S1 after mixing with part of the reaction product which is recycledthrough line R1: equalization of the feedstock composed of fresh feedand the fraction of recycled product therefore takes place in S1.

The feedstock leaving S1 reaches the feed-effluent exchanger E1 throughline 2, where the pump P1 is located.

The heat exchange between the feedstock and the product leaving thereactor A1 takes place in E1. The feedstock reaches the hydrogen feedingline 4 through line 3, and the mixture of hydrogen and feed is fed tothe reactor A1 through line 5: the oven F1 that heats the mixture to thereaction temperature situated on this line.

The hydrodeoxygenation product leaving the reactor reaches the exchangerE1 through line 6 and subsequently reaches the air exchanger a throughline 7 and the high-pressure separator H1 through line 7 a. Theseparation of the water SW, of the hydrocarbon fraction formed duringthe hydrodeoxygenation step and of the gaseous mixture prevalentlyconsisting of H₂, H₂S and CO₂, takes place in said separator H1. Thehydrocarbon fraction is sent through line 8 to the low-pressureseparator L1. The mixture of H₂, H₂S and CO₂ is sent through line 9 tothe acid gas treatment unit T1. The CO₂ and H₂S are absorbed in thisacid gas unit T1 by means of a specific absorbent and said absorbent isregenerated: a stream of hydrogen and a stream of CO₂ and H₂S aretherefore obtained at the outlet of the unit T1. The stream of hydrogenreaches line 11 through line 10, which feeds hydrogen deriving from aninternal source of the refinery, for example a reforming unit. Thecompressors K1 and K2 are present on line 10 and line 11 respectively.The hydrogen comes from line I which represents the hydrogen supplynetwork of the refinery. The resulting hydrogen flow reaches the feedline 3 through line 4. Lines 4 a and 4 b branch off from line 4, whichallow part of the hydrogen to be fed at different heights of the reactoralso obtaining quenching effect. The stream of CO₂ and H₂ at the outletof the unit T1 reaches the acid gas treatment unit T3 through line 12.

The separation of CO₂ and H₂S takes place in said acid gas unit T3through two absorption steps with specific absorbents and theirrespective regeneration: a stream of H₂S and a stream of CO aretherefore obtained at the outlet of the unit T3.

The stream of H₂S leaving T3 reaches the suction of the compressor K1,through the line R3, where the hydrogen flow of line 10 also arrives.

The stream of CO₂ leaving T3 is fed, through line 13, to the finalthermo-combustor of the Claus plant or is sent to one of the refineryovens suitably equipped with specific flues for the insertion of saidstream.

A further separation of water SW takes place in the low-pressureseparator L1, together with the separation from the hydrocarbon mixturefrom the Fuel Gas (FG), which is removed through line 41. Thehydrocarbon mixture is then sent to the fractionation column C1 throughline 14, on which the exchanger b is situated.

At the head of the fractionation column, line 29 brings a mixture of gasrich in propane and water to the condenser a1. Said mixture is fedthrough line 30 to an accumulator M, where it is carried out theseparation of the water, of the gas rich in propane which is removedthrough line 31 and of the liquid used as reflux in the fractionationcolumn C1, through line 32 and the pump P3. The gas rich in propane isindicated as LPG as it reaches the specifications of commercial LPGafter a washing with amines not shown in the FIGURE.

A hydrocarbon fraction substantially containing linear paraffins havinga number of carbon atoms which depends on the type of feed used, isseparated from the bottom of the fractionation column. Said hydrocarbonfraction reaches the vacuum dryer V1 by means of line 15 on which theexchanger c is situated. The flow of hydrocarbon product leaving V1,passes through line 16 into the pump d and then to the exchanger andthrough line 17, before being partly fed to the subsequent isomerizationsection through line L and partly recycled into the feed through lineR1.

The hydrocarbon product of the HDO section leaving the exchanger is fedthrough line L to the surge drum S2. The recycling line R2 of theisomerization product is inserted on line L and equalization of the feedis obtained in S2.

The feed leaving S2 reaches the exchanger E2 through line 18 on whichthe pump P2 is situated.

The heat exchange between the feed and the product leaving the reactorA2 takes place in E2. The feed reaches the hydrogen feeding line 20,through line 19, and the mixture of hydrogen and feed is fed throughline 21 to the reactor A2: the oven F2 which heats the mixture to thereaction temperature is situated on said line.

The isomerization product leaving the reactor A2 reaches the exchangerE2 through line 22 and subsequently reaches the air exchanger a2 throughline 23 and the high-pressure separator H2 through line 24. Theseparation of the water SW, of the isomerized hydrocarbon fractionformed during the isomerization step and of hydrogen takes place in saidseparator H2. The hydrocarbon fraction is sent through line 25 to thelow-pressure separator L2. The stream of hydrogen leaves the separatorH2 through line 26 and said line 26 joins the line X, whose function isto by-pass the acid gas treatment unit T2. T2 is part of thehydrodesulfurization unit which has been revamped and is not used in thenew HDO/ISO unit. The line X therefore allows the hydrogen flow not topass through T2 and joins line 27, through which the hydrogen flowreaches line 28, which feeds hydrogen deriving from an internal sourceof the refinery, for example a reforming unit. The compressors K3 and K4are present on line 27 and 28 respectively. The resulting hydrogen flowreaches the feed line 19 through line 20. The existing lines 20 a and 20b branch off from line 20, which allow part of the hydrogen to be fed atdifferent heights of the isomerization reactor also obtaining aquenching effect.

A further separation takes place in the low-pressure separator L2, ofthe water SW from the isomerized hydrocarbon mixture and from the FuelGas (FG) which is removed by means of line 33. The isomerizedhydrocarbon mixture is then sent to the fractionation column C2 by meansof line 34, on which the exchanger b2 is situated. An isomerizedhydrocarbon fraction is separated from the bottom of the fractionationcolumn. Said hydrocarbon fraction reaches the vacuum dryer V2 throughline 35 on which the exchanger s is situated. The flow of isomerizedhydrocarbon product, through line 36, is fed to the pump P3: the flowleaving the pump is partly recovered and partly recycled through line R2to the line L. The product recovered is a high-quality diesel of abiological origin (Green Diesel).

At the head of the fractionation column C2, the line 37 brings a mixtureof fuel gas and naphtha to the exchanger a3. Said mixture is fed throughline 38 to an accumulator M1. The Fuel Gas is separated from the naphthain said accumulator M1. The fuel gas is removed through line 39 whichjoins line 25.

The naphtha is partly recycled, through line 40, on which the pump P4 issituated, to the fractionation column C2, (line 40 a) and partlyrecovered as high-quality naphtha of a biological origin (GreenNaphtha).

A method has therefore been found for transforming hydrodesulfurizationunits into conversion units of mixtures of a biological origin based onfatty acid esters into fuel bases, whose main product is diesel anddiesel component, in addition to naphtha and LPG, with minimumvariations in the equipment already existing and a limited number ofsuitably selected substitutions and new installations.

The conversion unit of mixtures of a biological origin obtained with thetransformation method of the present invention is also an object of theinvention, as is also a HDO/ISO process carried out using saidconversion unit of mixtures of a biological origin.

A further object of the invention also relates to a HDO/ISO process forthe production of a hydrocarbon fraction, wherein said hydrocarbonfraction can be used as a fuel, or fuel component, starting from amixture of a biological origin containing fatty acid esters, andpossibly containing free fatty acids, which comprises the followingsteps:

(1) hydrodeoxygenation of the mixture of a biological origin;

(2) separation of the mixture resulting from step (1) into a hydrocarbonfraction and a gaseous mixture G comprising H₂, CO₂ and H₂S,

(3) hydroisomerization of the hydrocarbon fraction obtained in step (2),

(4) separation of the gaseous mixture G into a stream of hydrogen and agaseous mixture of CO₂ and H₂S,

(5) separation of the gaseous mixture of CO₂ and H₂S obtained in step(4) into a stream of H₂S and a stream of CO₂,

(6) feeding the stream of H₂S obtained in step (5) to thehydrodeoxygenation step.

The hydrogen obtained in step (4) is also re-fed to thehydrodeoxygenation step. The re-feeding of the H₂S to thehydroisomerization step allows the catalyst of said step to be kept inits sulfided form, and therefore active, without the necessity of havingto add further sulfiding agents, or in any case adding them in a reducedquantity.

The revamping method, object of the invention, the HDO/ISO unit obtainedusing the revamping method of the present invention, the HDO/ISO processusing said HDO/ISO unit, and the particular process comprising steps(1)-(6) indicated above, allow the production of diesel having excellentproperties (high octane index, optimum cold properties, high calorificvalue) and a stream of gas rich in propane which, after purificationwith amines, is in line with the specifications of commercial LPGobtained with the known methods, at the same time maintaining itsbiocomponent nature. A kerosene fraction and a naphtha fraction are alsoobtained, wherein said naphtha fraction can be used as such as gasolinebase, by upgrading its bio portions thanks to integration in thegasoline pool of refineries, or sent to reforming, thus contributing tothe synthesis of hydrogen to be used in the HDO/ISO process.

The invention claimed is:
 1. A biorefinery, comprising a production unitof a hydrocarbon fraction from a mixture of a biological origincomprising a fatty acid ester by means of hydrodeoxygenation andisomerization of the mixture of biological origin, wherein thebiorefinery is obtained from a refinery comprising a system comprisingtwo hydrodesulfurization units, (U1) and (U2), by means of a method forrevamping the refinery into the biorefinery, wherein each of thehydrodesulfurization units (U1) and (U2) comprises: ahydrodesulfurization reactor (A1) for the unit (U1) and ahydrodesulfurization reactor (A2) for the unit (U2), wherein each of thehydrodesulfurization reactors comprises a hydrodesulfurization catalyst;a heat exchanger that exchanges heat between a feedstock and effluent ofthe hydrodesulfurization reactor; a heating system that is differentfrom the heat exchanger that heats the feedstock upstream of thehydrodesulfurization reactor; and an acid gas treatment unit downstreamof the hydrodesulfurization reactor, comprising an absorbent (B) forH₂S, (T1) in the unit (U1) and (T2) in the unit (U2), the methodcomprising: installing a line L between the units (U1) and (U2) whichconnects them in series; installing a recycling line of the product forthe unit (U1) and optionally for the unit (U2), substituting thehydrodesulfurization catalyst in the hydrodesulfurization reactor (A1)with a hydrodeoxygenation catalyst; substituting thehydrodesulfurization catalyst in the hydrodesulfurization reactor (A2)with an isomerization catalyst; and installing a line to bypass the acidgas treatment unit (T2) of the unit (U2); and substituting the absorbent(B) in the acid gas treatment unit (T1) with a specific absorbent forCO₂ and H₂S.
 2. A process for producing a hydrocarbon fraction startingfrom a mixture of a biological origin comprising a fatty acid ester, theprocess comprising: (1) performing hydrodeoxygenation of the mixture ofa biological origin; and (2) performing hydroisomerization of a productresulting from the hydrodeoxygenation (1), after optionally purifying,wherein the process is effected with the biorefinery according toclaim
 1. 3. A HDO/ISO process for producing a hydrocarbon fractionstarting from a mixture of a biological origin comprising a fatty acidester and optionally a free fatty acid, the HDO/ISO process comprising:(1) performing hydrodeoxygenation of the mixture of biological origin,(2) separating a product resulting from the hydrodeoxygenation (1) intoa hydrocarbon fraction and a gaseous mixture G comprising H2, CO2 andH2S, (3) performing hydroisomerization of the hydrocarbon fractionobtained in the separating (2), (4) separating the gaseous mixture Ginto a stream of hydrogen and a gaseous mixture of CO₂ and H₂S, (5)separating the gaseous mixture of CO₂ and H₂S obtained in the separating(4) into a stream of H₂S and a stream of CO₂, and (6) feeding the streamof H₂S obtained in the separating (5) to the hydrodeoxygenation (1). 4.The process according to claim 3, wherein the hydrocarbon fractionproduced is a fuel or fuel component.
 5. The process according to claim4, wherein the fuel or fuel component is LPG, kerosene, diesel, ornaphtha.